It is known to attain an electrolysis by a so called solid polymer electrolyte (SPE) type electrolysis of an alkali metal chloride wherein a cation exchange membrane of a fluorinated polymer is bonded with a gas-liquid permeable catalytic anode on one surface and/or a gas-liquid permeable catalytic cathode on the other surface of the membrane (British Pat. No. 2,009,795, U.S. Pat. Nos. 4,210,501 and 4,214,958 and 4,217,401).
This prior art electrolytic method is remarkably advantageous as an electrolysis at a lower cell voltage because the electric resistance caused by the electrolyte and the electric resistance caused by bubbles of hydrogen gas and chlorine gas generated in the electrolysis can effectively be decreased. This has been considered to be difficult to attain in the electrolysis with cells of other configurations.
A high percentage of hydrogen in chlorine poses problems in chlorine liquefaction processes. Extra steps are required to prevent the formation of dangerous gas mixtures. The hydrogen problem is a severe drawback if a cost effective method to reduce hydrogen percentage cannot be identified.
The anode and/or the cathode in the prior art electrolytic cell are bonded on the surface of the ion exchange membrane so as to be partially embedded. The gas and the electrolyte solution are readily permeated so as to remove, from the electrode, the gas formed by the electrolysis at the electrode layer contacting the membrane. That is, there are few gas bubbles adhering to the membrane after they are formed. Such a porous electrode is usually made of a thin porous layer which is formed by uniformly mixing particles which act as an anode or a cathode with a binder. U.S. Pat. No. 4,822,544 to Coker et al, which is herein incorporated by reference, discloses a method of fabricating a membrane-electrode structure. However, it has been found that when an electrolytic cell having an ion exchange membrane bonded directly to the electrode is used, the anode in the electrolytic cell is brought into contact with hydroxyl ions which migrate back from the cathode compartment, and accordingly, both chlorine resistance and alkaline resistance for anode material are required for this prior art method and an expensive material must be used. When the electrode layer is directly bonded to the ion exchange membrane, a gas is formed by the electrode reaction between an electrode and membrane and certain deformation phenomenon of the ion exchange membrane causes the characteristics of the membrane to deteriorate. In such an electrolytic cell, the current collector for the electric supply to the electrode layer which is bonded to the ion exchange membrane, should closely contact the electrode layer. When a firm contact is not obtained, the cell voltage may be increased. Therefore, the cell structure for securely contacting the current collector with the electrode layer according to the prior art is disadvantageously complicated.
Additionally, in chlor-alkali electrolyzers where the cathode is directly bound to the membrane there is permeation of hydrogen through the membrane which enters the anolyte compartment and mixes with the chlorine. High percentages of hydrogen are then found in the chlorine so as to cause problems in the liquefaction process. Prior means for reducing the hydrogen percentage includes 1) the use of a platinum black layer on the anode side of the membrane, 2) the use of a layer (e.g. Ag) less electroactive than the electrode layer itself between the membrane and the electrode, 3) the use of thickened membranes, and 4) the use of a membrane with a lower permeation rate for hydrogen permeation. These methods have proved to be expensive and ineffective.
Perfluoro membranes which are used as membranes for electrolysis reactions usually have fairly low water contents. As compared with conventional ion exchangers with same amount of water contents, the conductivity of the perfluoro membranes are abnormally high. This is because of phase separation existing in the perfluoro ionic membranes. The phase separation greatly reduces the tortuosity for sodium ion diffusion. The hydrogen diffusion path is the aqueous ionic region and the amorphous fluorocarbon region. Therefore, the tortuorsity experienced by the hydrogen molecules is also low for the phase-segregated fluorocarbon membranes as compared with conventional hydrocarbon ionic membranes.
The phase-segregation characteristics of the fluorocarbon membranes provides the high migration rates for sodium ions, thus relatively lower ionic resistivity is also the cause for the high hydrogen diffusion rates and the resulting high percentage of hydrogen in chlorine. Moreover, the high permeation rate of hydrogen is even more enhanced by the high solubility of hydrogen in the fluorocarbon membranes because of the hydrophobic interaction between hydrogen molecules and the fluorocarbon chains. Therefore, reducing hydrogen permeation rates by increasing the thickness of the membranes or modifying the structure of the membranes would not be very effective because the sodium migration rate would be reduced as one tries to reduce the hydrogen diffusion rate; and the tortuosity effect is difficult to introduce because of the phase separation.
A retardation layer is defined as a layer between the electrode layer and the membrane to retard hydrogen permeation. Any kind of layer can have a certain effect to retard hydrogen permeation as long as it is (1) inactive for electrolytic hydrogen generation, and (2) flooded. The latter requirement is also important for low resistance (i.e., lower voltage and good performance). With these considerations a layer of a blend of inert solid particles (usually inorganic) and binders (usually organic) would serve the purpose best.
The need for a binder is obvious: the binder can (1) bind the components in the barrier layer together and also (2) provide the necessary adhesion between the retardation layer and the electrode layer and that between the retardation layer and the membrane. The function of the solid particles is also two fold: (1) providing the physical strength to the barrier layer so that there is very limited interpenetration between different layers during fabrication, and (2) forming an agglomerate with the binder.
The reason that the retardation layer is better than the membrane itself in retarding hydrogen permeation is because (1) it allows hydroxide and sodium ions to migrate at a faster rate so relatively small voltage penalty has to be paid. On the other hand, in the membrane, sodium ion diffusion is slowned down by the coulombic interaction exerted by the sulfonate or carboxylate groups. The situation is even worse when the membrane is immersed in strong caustic solution as in the clor-alkali membrane. This is particularly severe for the carboxylic membranes. Ion pairing between sodium ions and carboxylate groups and hydroxide ions is believed to be the cause for the very slow diffusion rate when membrane dehydration occurs under this condition. The solubility of hydrogen is much lower in caustic solution than in the membrane, so the permeation rate (the product of diffusion coefficient and solubility) of hydrogen can be reduced by a larger factor compared with that of the sodium and hydroxyl ions. By introducing the blend of inorganic particles and binder and with the necessary morphology, the resistance of the caustic solution is increased. The ratio of the resistivity of the porous medium saturated with electrolyte, R.sub.p, to the bulk resistivity of the same electrolyte solution, R.sub.b is commonly called "formation resistivity factor", EQU F=R.sub.p /R.sub.b =X/O.
This equation describes the relationship between F and "electric tortuosity", X, and O, the porosity. X is different from hydraulic tortuosity which takes into account the fact the effective path length experienced the diffusing species is increased by the presence of impermeable blocking materials. On the other hand, X also takes into account the special effects due to convergent-divergent nature of the capillaries, called constrictedness, besides the hydraulic tortuosity.
Since conductivity is proportional to diffusion rate of the ionic species, formation resistivity factor is also related to diffusion rate in the porous medium, D.sub.p, and the diffusion rate in the bulk electrolyte, D.sub.b, by the following equation: EQU F=R.sub.p /R.sub.b =D.sub.b /D.sub.p
U.S. Pat. No. 4,832,805 to La Conti et al discloses a membrane-electrode assembly for electrolysis processes with multiple layers having different overvoltages. There is a layer attached to the membrane which has a higher overvoltage for the electrolysis process than the electrode attached to its upper surface. The intermediate layer comprises a polymeric binder such as tetrafluoroethylene and conductive metal or carbon particles.